The cell wall responses to anaerobiosis
The yeast cell wall represents a dynamic organelle that modifies in composition and functionality in response to external stimuli (Klis, 1994; Cabib et al., 1997; Caro et al., 1998; Abramova et al., 2001a; Boorsma et al., 2004; Rhymes and Smart, 2001; Smart, 2003; Lesage and Bussey, 2006). Under aerobic growth conditions, Cwp1p and Cwp2p, encoded by the genes CWP1 and CWP2, respectively, are the most abundant cell wall man- noproteins and are involved in cell wall biosynthesis during cell replication (van der Vaart et al., 1995). During environmental stress, the composition of the cell changes, resulting in the increase in cell wall mannoproteins other than Cwp1p and Cwp2p (Abramova et al., 2001a). The most extensive response is the induction of several homologous mannoproteins (Dan1p, Tip1p, Tir1p, and Tir2p) during anaerobic growth. The genes encoding these proteins are referred to as the DAN/ TIR genes. This group of genes also includes genes for five other mannoproteins, designated Dan2p, Dan3p, Dan4p, Tir3p, and Tir4p. The Tir1p, Tir3p, and Tir4p proteins are required for anaerobic growth, with knockout yeast becoming arrested at G1 in anaerobic conditions (Abramova et al., 2001a). During N2-induced anaerobic adaptation, the CWP1 and CWP2 genes are down-regulated and the DAN/ TIR genes are up-regulated, allowing an extensive remodelling of the yeast cell wall (Abramova et al., 2001a). The DAN genes DAN1 and DAN4 are expressed within an hour of a N2-induced anaerobic shift, and DAN2 and DAN3 are expressed after approximately three hours (Abramova et al., 2001b). Anaerobic expression of the DAN/TIR genes depends on the binding of Upc2p to an upstream regulatory element. In addition, Ecm22p plays a role in the expression of DAN2 and DAN3 but not in the expression of any other DAN/TIR genes. Davies and Rine (2006) identified sterol level as the primary regulator of Upc2p. Furthermore, they showed that DAN2 and DAN4 were activated by Upc2p solely in response to sterol depletion rather than haem depletion, whereas DAN1 responded to both haem and sterols (Davies and Rine, 2006). Repression of the DAN genes results from the combined action of at least six repression factors - Mox1p, Mox2p, Rox1p, Rox7p, Tup1p, and Ssn6p (Abramova et al., 2001b). Some of the DAN genes are also repressed by Mot3p, a repressor that is induced by haem and repressed in anaerobic cells by Hap1p, in parallel with the Rox1p repressor. Abramova et al. (2001a) concluded that Mot3p, which is known to function as an activator or repressor of different genes through the same binding site, is an important activator of CWP2, presumably through the three Mot3p sites in the promoter. Dan1p, along with the putative ABC transporter (ATP-binding-cassette transporter) Aus1p, plays an essential role in anaerobic sterol uptake (Alimardani et al., 2004). No functions associated with anaerobiosis have been assigned to the products of DAN2, DAN3 or DAN4, despite their responsiveness to this condition.
The remodelling of the cell wall is important in the brewing context, as it has been demonstrated that the flocculation potential of brewing yeast strains is altered by growth in aerobic and anaerobic conditions (Lawrence et al., 2011; Lawrence and Smart, 2011).